The present invention relates to a scanning optical device for scanning a laser beam by deflecting the laser beam with a deflection mirror, and particularly to a scanning optical system having a micro deflection mirror which functions as a deflector for deflecting the laser beam by producing sinusoidal vibrations.
In general, a scanning optical device for imaging devices such as a laser copying device or a laser printer is configured to deflect a laser beam emitted by a light source while on-off modulating the laser beam in synchronization with a modulating signal generated based on image data so that the laser beam scans on a drawing surface in a predetermined scanning direction. The scanning optical device further moves the drawing surface in a direction perpendicular to the predetermined scanning direction of the laser beam so that a required image can be formed on the drawing surface. Hereafter, the predetermined scanning direction in which the laser beam scans on the drawing surface by deflection by the deflector is referred to as a main scanning direction, and the direction in which the drawing surface is moved perpendicularly to the main scanning direction is referred to as an auxiliary scanning direction.
In the above mentioned scanning optical device for imaging devices, a polygonal mirror having a plurality of mirror surfaces is employed as a deflector for deflecting the laser beam. Further, an fθ lens (or an fθ lens group) functioning as a scanning optical system is located on a rear side with respect to the polygonal mirror so that the laser beam scans on the drawing surface at a constant speed. However, such a scanning optical device needs to have a motor unit for rotating the polygonal mirror. Use of such a motor unit needs relatively large space in the scanning optical device, and thereby increases cost of the scanning optical device.
Japanese Patent Provisional Publication No. 2002-182147 (hereafter, referred to as JP2002-182147A) discloses a scanning optical device employing a micromirror device functioning as a deflector. By employing a micromirror device, cost reduction and downsizing of the scanning optical device can be achieved. Such a scanning optical device employing a micromirror device also needs use of a scanning optical system as in the case of the scanning optical device employing the polygonal mirror.
In the scanning optical device disclosed in JP2002-182147A, the micromirror device deflects an incident laser beam periodically by causing a mirror surface to produce sinusoidal vibrations around a predetermined axis. It is understood that if such a micromirror device is used to deflect the incident laser beam, the speed of a beam spot moving on the drawing surface varies such that the speed becomes slower at a point near to edges of a scanning range. Therefore, in this case, the scanning optical system needs to have, in place of an fθ property, an arcsine property where the amount of distortion becomes larger at a point closer to edges of the scanning optical system so that the speed of the beam spot can be kept constant within the entire scanning range.
In order to downsize a scanning optical device by employing a micromirror device, a scanning optical system also needs to be downsized. However, in general, the smaller the size of a scanning optical system becomes, the larger the amount of aberration becomes. Therefore, it is necessary to downsize a scanning optical system adapted for use with an existing galvanometer mirror which produces sinusoidal vibrations similarly to a micromirror device while keeping the amount of aberration such as a curvature of field in an acceptable range.